I'm currently working with the open-source OpenToonz (released by Studio Ghibli: https://opentoonz.github.io/e/), but the ECMAScript API is not very robust.
I tried Blender3D, which has a robust Python API, but it renders extremely slowly, even with raytracing off.
Youtuber 3Blue1Brown has a specialized math animation toolkit (https://github.com/3b1b; example here: https://www.youtube.com/watch?v=AmgkSdhK4K8), but it's lacking in tutorials and doesn't seem too suited for my purposes.
Unfortunately it is not free or open source. However it is extremely powerful. It has python and it has a robust API. It has daily builds and top tier support! Also lively communities over at
forums.odforce.net
and
https://www.sidefx.com/forum/
and tons and tons of tutorials.
(source: Me, I've been a huge Houdini fan for 20 years)